Mass Transport Method Research Articles

Two-dimensional adaptive Eulerian-Lagrangian method for mass transport with spatial velocity distribution

A Eulerian-Lagrangian scheme is used to solve the two-dimensional advection-dispersion equation. Concentration and its partial differential operator are decomposed into advection and dispersion terms. Thus, advection is formally decoupled from dispersion and solved by continuous forward particle tracking. Dispersion is handled by implicit finite elements on a fixed Eulerian grid. Translation of steep gradients of concentration in advection-dominated flow regimes, is done without numerical distortion. Continuous spatial distribution of velocities are evaluated by using Galerkin's approach in conjunction with Darcy's law based on hydraulic input data from each element. The method was implemented on coarse FE grid with linear shape functions, demonstrating no over/under shooting and practically no numerical dispersion. Simulations, covering a wide range of Peclet numbers, yield high agreement with analytic and practical results.

Electrophoretic mobility of alumina, titania and their mixtures in aqueous dispersions

The electrophoretic mobility of commercial alumina and titania powders and their mixtures (ratio of alumina:titania mixtures (by weight) 1:2 and 4:1) dispersed in 10 −3 mol dm −3 KNO 3 was determined as a function of pH in the range pH 3–11, using both microelectrophoresis and mass transport methods. The results are in good agreement with the previous experimental results. The value of the isoelectric point (IEP) for alumina and titania was found to be at pH9 and 5.5, respectively. It appears that the dispersion concentration has no significant effect on the electrophoretic mobility versus pH. The results also suggest that the addition of titania to alumina has lowered the IEP value from pH ∼9 to ∼8.4 and the addition of alumina to titania has increased the IEP value from pH ∼5.5 to 6. The electrophoretic mobility versus pH profiles for the mixtures of alumina and titania powders appear to represent the electrokinetic behavior of both alumina and titania powders; however, the shape of the electrophoretic mobility versus pH profiles and the position of the isoelectric point appear to be governed by the powder with a large total surface area in the mixture, but not necessarily the weight.

The determination of cholesteric pitch from the diffusion profile A new experimental approach

Abstract The optical microscopic mass transport (OMMT) method was utilized to determine the cholesteric pitch values of the mixtures of mesogenic and non-mesogenic chiral species in nematic materials. The cholesteric pitch was determined by transient analysis of the diffusion profile established by allowing an initial cholestric composition (solute) to diffuse under semi-infinite linear boundary conditions into an oriented thin film of its corresponding nematic solvent. During the steady-state diffusion, where the initial concentration (pitch) remains constant, the transient analysis of the profile with a polarization microscope exhibits a maximum number of pitch discontinuity domains. The evaluation of the unknown pitch was carried out by extrapolation of the diffusion pitch gradient to the original diffusion source by using the exact relation between the pitch and the film thickness. The method is experimentally simple; at constant temperature and pressure, determination of the pitch depends only on the...

The Critical Pitch-Thickness Relation at the Cholesteric-Nematic Phase Transition by the Optical Mass Transport Method

Abstract The optical mass transport (OMT) method was utilized to study the cholesteric pitch as a function of thickness at the cholesteric-nematic phase transition in a wedge-type diffusion cell at two different geometries. We obtained a linear relation between the cholesteric finger print pitch (Pc) and the film thickness (Lc) at the critical cholesteric-nematic boundary layer. The measurements were done by direct observation via an optical microscope. Due to the presence of the concentration gradient, the surface anchoring is weakened and large pitch values within the range of Lc/Pc ≤ 0.15 were established. The thickness gradient as well as the concentration gradient showed systematic linear effects on the data. We found a qualitative agreement between the present experimental data and that of a previous theory.

Diffusion study of a polymer–diluent system using the optical mass transport method

AbstractThermotropic polymers and low‐molecular‐weight mesogens share many common textural features. This circumstance is exploited to determine diffusion coefficents in a system consisting of a cholesteric polymer and a low‐molecular‐weight nematogen using the recently developed optical mass transport method. The self‐diffusion coefficient and its concentration dependence were determined by using a distance–time approach, whereas the time dependence of the mutual diffusion coefficient was evaluated by a conventional concentration–distance analysis of the diffusion profile. Comparison with literature data indicates satisfactory agreement. The coefficient of the scaling law relating the self‐diffusion coefficient and concentration is in accord with the value predicted by de Gennes for semidilute polymer solutions.

Time Dependence of Impurity Diffusion in a Nematic Liquid Crystal

Abstract In the optical method of mass transport in nematic liquid crystals, the diffusion time was employed as a variable experimental parameter. Within the time range where concentration gradient is large, we found that the average diffusion coefficient of the system to increase with time. This is due to an overall material flow coupled with Fickian diffusion at the vicinity of the solute-solvent boundary.

Experimental verification of an estimation method for mass transport in magnetoelectrolysis

Experimental data are presented to show that a previously proposed analytical method for estimating mass transport rates in a magnetoelectrolytic cell is of reasonable accuracy in moderate magnetic fields.

Experimental verification of an estimation method for mass transport in magnetoelectrolysis

Experimental data are presented to show that a previously proposed analytical method for estimating mass transport rates in a magnetoelectrolytic cell is of reasonable accuracy in moderate magnetic fields.

The mass transport method for determining electrophoretic mobilities

An apparatus embodying the Hittorf determination of electrophoretic mobilities has been evaluated. It was shown to yield meaningful data only for rather stable suspensions and even then only if some quite stringent precautions are taken. Effects limiting its performance are: particle settling, electrodic contamination, electrode gassing and Joule heating. Because of these problems the Hittorf method in this form is not recommended except for concentrated suspensions where other methods fail.

Surface diffusion of carbon on (111) platinum

An Auger electron spectrometric technique has been employed to measure the diffusivity of carbon atoms on an otherwise clean Pt(111) surface. The technique used was essentially a classical mass transport method, in which the rate the migration of carbon atoms out of a circular surface region initially containing a monolayer of adsorbed carbon was measured by Auger electron spectrometry. The diffusivity D can be characterized by an Arrhenius equation D=D0exp(—ΔH*/RT) in which D0=0.2 cm2/s and ΔH*=31±4 kcal/mol. Additional data are presented to demonstrate the self-consistency of the experimental method used.

Reaction Kinetics of Uranium and Deuterium

A uniform and adherent layer of β-UD3 was formed on uranium when uranium reacted with deuterium in the temperature range 175–350°C and pressure range 100–10 270 torr. The rate of reaction was controlled by diffusion of deuterium across this product layer. Reproducible results were obtained only with D2 of very high purity because low-level impurities had a profound effect on the rate of reaction. It was concluded that molecular diffusion with sorption was the predominant method of mass transport, but atomic diffusion may also be occurring. The activation energy for molecular diffusion with Langmuir sorption of D2 on UD3 was determined to be 9.8 ± 0.5 kcal/mole D2. The activation energy for atomic diffusion was determined to be 4.4 ± 0.3 kcal/g-atom of D.

Surface diffusion of oxides (A review)

The existing data for surface diffusion on oxides have been gathered by two types of measurements: mass transport (grain boundary grooving and scratch smoothing) and tracers. Mass transport methods yield an effective surface diffusion coefficient, without being able to determine directly whether metal ions or oxide ions control the rate of mass transport, while tracer studies measure the diffusion of a single ion. The analysis of the experimental data for both methods must take into account volume diffusion and evaporation-condensation in addition to surface diffusion.Mass transfer measurements on aluminum oxide, magnesium oxide, and uranium dioxide indicate that surface diffusion is dominant over volume diffusion at high temperatures for these oxides. The only tracer measurements of surface diffusion on an oxide to date are measurements on uranium dioxide. Reanalysis of the tracer results indicates the volume diffusion contribution to the tracer motion may be considerable.Small amounts of impurities can greatly affect the apparent surface diffusion coefficient. Departures from stoichiometry can affect the mechanism of material transport by altering the absolute value of the surface diffusion coefficient and its value relative to the volume diffusion coefficient. The validity of models of surface diffusion on oxides is examined in relation to the existing experimental data.

Progress in Control of Vehicle Emissions

The control of emissions from motor vehicles is reviewed including: the contribution of vehicles to total emissions of air pollutants; regulations and emission standards for pollutants of concern; methods and equipment required; procedures for certification, maintenance, and surveillance of control equipment; outlook for future regulations and controls; development of pollution-free or low-pollution vehicles, including new types of power sources; and use of mass transportation methods in central cities.